Electric vehicles and hybrid vehicles are equipped with a high-voltage battery as a power source for driving a motor. The high-voltage battery like this obtains a high voltage by series-connecting multiple cells together in each of its rechargeable batteries (storage batteries) such as nickel-metal hydride batteries or lithium-ion batteries.
In addition, the entire rechargeable battery is charged or discharged with the same level of power. For this reason, if the level of deterioration differs in one rechargeable battery, the rechargeable battery tends to be over-charged or over-discharged. With this taken into consideration, the charged state of each cell needs to be checked in order to prevent overcharge or overdischarge of the rechargeable battery. To this end, the multiple cells (for example, 55 cells) are divided into, for example, 5 blocks (each consisting of 11 cells); and the voltage of each block is measured in real time by use of a voltage detection IC provided to the block.
Furthermore, in a case where the high-voltage battery is used for a long time while repeatedly charged and discharged, or in a case where the high-voltage battery is left unused for a long time, the state of charge varies from one rechargeable battery to another. This makes the usable capacity of the high-voltage battery diminish, and makes it no longer possible to obtain a sufficient level of electric power from the high-voltage battery. Against this background, proposals have been made on a cell voltage equalizer configured to equalize the output voltages from the respective cells, as disclosed in Japanese Patent Application Publication No. 2003-189490 (Patent Document 1) and Japanese Patent Application Publication No. 2002-101565 (Patent Document 2).
For the purpose of equalizing the output voltages from the respective cells in each of the rechargeable batteries mounted on an electric vehicle or a hybrid vehicle, the cell voltage equalizer like this detects the output voltages from the respective cells, and discharges the output voltages from cells each having a high output voltage in order to make their output voltages equal to the lowest output voltage, while the ignition is off (see Japanese Patent Application Publication